1. Field of the Invention
The present invention generally relates to a radar apparatus which is mounted on an automobile and/or installed on a road facility so as to detect an object (obstacle and traveling vehicle) appeared on a road. That is, the present invention is related to an FM-CW radar apparatus for detecting a beat signal of an FM transmission signal wave, which is produced by a reflection wave reflected from an object, and for analyzing a frequency component of the beat signal so as to calculate both a distance and a velocity-of the object. More specifically, the present invention is directed to an FM-CW radar apparatus capable of reducing a measuring time duration by xc2xd, required to detect an object in such a manner that both an FM modulation wave along a frequency-up direction and an FM modulation wave along a frequency-down direction are simultaneously transmitted, and then, beat frequency components with respect to the respective FM modulation waves are analyzed so as to calculate a distance and a velocity of an object.
2. Description of the Related Art
In general, as described in, for instance, Japanese Patent Application Laid-open No. 63-275976 (Japanese Patent No. 2550574), FM-CW type radar apparatus have be widely used as radar apparatus designed for automobiles.
FIG. 7 and FIG. 8 are explanatory diagrams for explaining a basic idea of a conventional FM-CW radar apparatus. That is, FIG. 7 represents a change in reception frequencies and a change in beat frequencies in the case that a signal is transmitted to a stationary object, whereas FIG. 8 shows a change in reception frequencies and a change in beat frequencies in such a case that a signal is transmitted to a moving object.
In these drawings, transmission frequencies of transmission signals to objects (targets) which should be detected; reception frequencies of reflection signals reflected/received from the objects; and the respective beat frequencies xe2x80x9cfbuxe2x80x9d and xe2x80x9cfbdxe2x80x9d obtained when the signal is frequency-modulated along the up direction, and the signal is frequency-modulated along the down direction are represented in the form of waveforms as a relationship with respect to time xe2x80x9ctxe2x80x9d, respectively.
In FIG. 7, a carrier wave (FM modulation wave) having a center frequency xe2x80x9cf0xe2x80x9d is transmitted by way of an FM modulation method by which the carrier wave is repeatedly changed to a triangular shape.
A triangular-shaped wave indicated by a solid line in FIG. 7 shows a relationship between the frequency of the transmission signal and the time xe2x80x9ctxe2x80x9d. Another triangular-shaped wave indicated by a broken line shows a relationship between a reception signal and the time xe2x80x9ctxe2x80x9d. This reception signal is reflected/received from, for example, an object located at a distance xe2x80x9cRxe2x80x9d. This triangular-shaped wave is delayed by such a time duration defined by that the transmission signal has been transmitted, and the reflection signal is received.
In this case, assuming now that frequencies of beat signals constructed of frequency differences between transmission signals and reception signals during frequency-up modulation and also frequency-down modulation are selected to be xe2x80x9cfbuxe2x80x9d and xe2x80x9cfbdxe2x80x9d, the respective beat frequencies xe2x80x9cfbuxe2x80x9d and xe2x80x9cfbdxe2x80x9d are expressed by the below-mentioned equation (1).
fbu=xe2x88x92fr
fbd=frxe2x80x83xe2x80x83(1)
It should be noted that in the above-described equation (1), symbol xe2x80x9cfrxe2x80x9d shows a beat frequency caused by a reflection signal which is reflected from a stationary object located at a distance of xe2x80x9cRxe2x80x9d. This beat frequency is given by the below-mentioned equation (2) by employing a repetition frequency xe2x80x9cfmxe2x80x9d of an FM signal (FM modulation wave), a frequency shift width xe2x80x9cxcex94Fxe2x80x9d of the FM signal, and a light velocity xe2x80x9ccxe2x80x9d.
fr=4xc3x97Rxc2x7fmxc2x7xcex94F/cxe2x80x83xe2x80x83(2)
Based upon this equation (2), the distance xe2x80x9cRxe2x80x9d is calculated in accordance with the below-mentioned equation (3).
R=frxc2x7c/(4xc3x97fmxc2x7xcex94F)xe2x80x83xe2x80x83(3)
On the other hand, in the case that an object is moved, both a frequency change in transmission signals and a frequency change in reception signals with respect to time, which are caused by the Doppler effect, are as indicated in FIG. 8.
In general, a Doppler frequency xe2x80x9cfvxe2x80x9d is given by the following equation (4).
fv=2xc3x97vrxc2x7f0/cxe2x80x83xe2x80x83(4)
In this equation (4), symbol xe2x80x9cvrxe2x80x9d indicates a velocity (speed) of the object. This velocity xe2x80x9cvrxe2x80x9d of the object may be given by the below-mentioned equation (5).
vr=fvxc2x7c/(2xc3x97f0xe2x80x83xe2x80x83(5)
Also, in FIG. 8, the beat frequencies xe2x80x9cfbuxe2x80x9d and xe2x80x9cfbdxe2x80x9d which are caused by reflection signals reflected from such an object which is approached are defined based upon the below-mentioned equation (6), namely are equal to such values obtained by adding the Doppler frequency xe2x80x9cfvxe2x80x9d to the beat frequencies obtained in the case of the stationary object.
fbu=xe2x88x92fr+fv
fbd=fr+fvxe2x80x83xe2x80x83(6)
In accordance with the above-described equation (6), both the Doppler frequency xe2x80x9cfvxe2x80x9d and the beat frequency xe2x80x9cfrxe2x80x9d are expressed based upon the below-mentioned equation (7).
fv=(fbd+fbu)/2
fr=(fbdxe2x88x92fbu)/2xe2x80x83xe2x80x83(7)
The above-explained equation (7) is substituted for the above-mentioned equations (3) and (5), so that both the distance xe2x80x9cRxe2x80x9d of the object and the velocity xe2x80x9cvrxe2x80x9d of this object may be calculated by employing the measured beat frequencies xe2x80x9cfbuxe2x80x9d and xe2x80x9cfbdxe2x80x9d as follows.
R=(fbdxe2x88x92fbu)xc2x7c/(8xc3x97fmxc2x7xcex94F)
vr=(fbd+fbu)xc2x7c/(4xc3x97f0)xe2x80x83xe2x80x83(8)
In this case, resolution xe2x80x9cxcex94vxe2x80x9d of the velocity xe2x80x9cvrxe2x80x9d is determined based upon analyzable minimum frequencies of the beat frequencies xe2x80x9cfdxe2x80x9d and xe2x80x9cfrxe2x80x9d. Since the repetition frequency of the FM modulation wave is equal to xe2x80x9cfmxe2x80x9d, this resolution xe2x80x9cxcex94vxe2x80x9d of the velocity xe2x80x9cvrxe2x80x9d may be determined for either a frequency ascent time period or a frequency descent time period (=2xc3x97fm) one time.
In other words, the resolution xe2x80x9cxcex94vxe2x80x9d of the velocity xe2x80x9cvrxe2x80x9d may be expressed by the following equation (9).
xcex94v=fmxc2x7c/f0xe2x80x83xe2x80x83(9)
On the other hand, in the case that a plurality of objects are present on a road, a plurality of beat signals are produced during the frequency-up modulation and also during the frequency-down modulation, the total number of which correspond to the total number of these objects.
In this case, in order to detect only a specific object, a beat signal of the relevant object is selected from the plurality of beat signals. Then, both the distance xe2x80x9cRxe2x80x9d and the velocity xe2x80x9cvrxe2x80x9d of this specific object are calculated from the respective beat signals during both the frequency-up modulation and the frequency-down modulation.
In order to select a combination of beat signals, such data as magnitudes of signal components of these beat signals may be used as reference purposes.
In other words, such beat signals whose signal levels are substantially equal to each other are selected from signals obtained during the frequency-up modulation and the frequency-down modulation, and then, the selected beat signals are combined with each other.
On the other hand, while an interval control operation between successively-driven automobiles is carried out, such a fact is known. That is, a change in vehicle drive speeds rather than a change in the above-described intervals between the successively-driven vehicles may give a large influence to a comfortable driving condition.
As a consequence, in order that a vehicle speed of the own vehicle is smoothly controlled in response to a relative speed with respect to a preceding vehicle so as to improve such a comfortable driving condition, this relative speed should be measured in high resolution.
In the above-explained radar apparatus, as previously described, in order to improve the resolution xe2x80x9cxcex94Vxe2x80x9d of the velocity xe2x80x9cvrxe2x80x9d, the repetition period of the modulation should be set to the longer repetition period.
However, when the repetition period is made longer, the data updating period is lowered directly proportional to this long repetition period. As a result, there is a problem that the response characteristic of the radar detection operation is lowered.
In particular, generally speaking, in an automobile radar apparatus, while a radar beam is scanned, distances along a plurality of directions are measured so as to recognize a direction of a preceding vehicle. When data is updated one time, distances must be measured plural times in correspondence with a scanning direction.
As previously described, in the FM-CW radar apparatus, while the beat signal between the reflection signals is measured during the two modulation periods (namely, frequency-up modulation period and frequency-down modulation period), both the distance xe2x80x9cRxe2x80x9d and the velocity xe2x80x9cvrxe2x80x9d are measured. As a result, the time duration required for measuring the velocity xe2x80x9cVrxe2x80x9d must become two times longer than the time duration required for measuring the Doppler signals.
For example, in such a radar apparatus having a center frequency xe2x80x9cf0xe2x80x9d of 76.5 GHz, in order to measure a radar signal in resolution of such a relative speed (=0.5 km/h), such a time duration of xe2x80x9c1/fm (=c/(f0xc2x7xcex94v)=0.028s)xe2x80x9d per one direction is required. Thus, a time duration of xe2x80x9c5xc2x71/fm (=0.14 s)xe2x80x9d per 5 directions is needed, which is five times longer than the first-mentioned time duration of xe2x80x9c1/fmxe2x80x9d.
Normally, in order to measure an angle with higher precision, or to measure a wider range, the total number of scanning directions must be increased. This implies that measuring time duration is increased. In other words, this implies that a time duration required for a single scanning operation is increased. As a result, this may induce that the control response characteristic and the control performance of the FM-CW radar apparatus are deteriorated.
For example, in the conventional radar apparatus described in the above-explained Japanese Patent Application Laid-open No. 63-275976 (Japanese Patent No. 2550574), while both the upper side band signal and the lower side band signal are transmitted at the same time, the frequencies of which are repeated along the ascent direction and also the descent direction within a constant time period, both the distance xe2x80x9cRxe2x80x9d and the velocity xe2x80x9cvrxe2x80x9d of the object are measured from the frequency differences in the reflection signals.
The above-explained conventional radar apparatus is not directed to shortening of the measuring time. However, as a result, since the signal modulation is performed one time in order to measure the velocity, this conventional radar apparatus is in principle arranged in such a manner that the measuring time may be reduced by xc2xd.
However, in this conventional radar apparatus, the reference oscillation signal (carrier wave) is mixed with the frequency modulation signal by the up-converter, and thereafter, both the upper side band signal and the lower side band signal are employed as the local signal. This local signal is used to extract the beat signals of the reflection waves and also the transmission waves. As a consequence, while the basic wave is suppressed, the upper side band signal must be completely separated from the lower side band signal.
To the contrary, in the millimeter band having the center frequency of 76 GHz employed in an automobile radar, the maximum occupied bandwidth is allowed only up to 1 GHz. Also, such a filtering technique could not be so far established, by which a practically operable filter having a sharp cut-off characteristic in the millimeter band is constructed. As a result, the above explained radar apparatus described in Japanese Patent Application Laid-open No. 63-275976 cannot be practically realized.
As described above, in the conventional FM-CW radar apparatus, when the repetition periods of the modulation operations are set to such long repetition periods in order to measure the relative velocity between the object and this radar apparatus in better resolution, the following problem may occur. That is, the data updating time period is lowered, so that the detection response characteristic is lowered.
Also, in such a case that this conventional FM-CW radar apparatus is applied to control the interval between the successively-driven vehicles, when the speed changes are controlled in the suppression mode in order to improve the comfortable driving condition, there is such a problem that the lengthy time is required to measure the velocity, and therefore, both the control response characteristic and the control performance would be lowered.
Also, as explained in the conventional radar apparatus of Japanese Patent Application Laid-open No. 63-275976 (Japanese Patent No. 2550574), in such a case that both the upper side band signal and the lower side band signal are transmitted at the same time, the frequencies of which are repeated along the ascent direction and also the descent direction within a constant time period, both the upper side band signal and the lower side band signal should be completely separated from each other, while completely suppressing the basic wave signal. Therefore, there is such a problem that this radar apparatus cannot be realized in such a frequency band which is used in the automobile radar apparatus.
The present invention has been made to solve the above-described problems, and therefore, has an object to provide such an FM-CW radar apparatus capable of reducing a physically-required radar signal measuring time duration by xc2xd, while measuring means for measuring both a distance of an object and a velocity thereof by way of both a frequency-up modulation and a frequency-down modulation is applied thereto.
To achieve the above-explained object, an FM-CW radar apparatus according to the present invention is characterized by comprising: transmission means for separately producing a first FM modulation wave along a frequency-up direction and a second FM modulation wave along a frequency-down direction to transmit both the first FM modulation wave and the second FM modulation wave at the same time; reception means for receiving reflection waves reflected from an object, which are caused by the first and second FM modulation waves; beat signal detection means for detecting a first beat signal and a second beat signal in a separate manner between the reflection waves and the first/second FM modulation waves; and an analysis apparatus for analyzing frequency components of the first and second beat signals so as to measure a distance of the object and also a velocity of the object.
Also, the FM-CW radar apparatus according to the present invention is characterized in that a frequency of the first FM modulation wave and a frequency of the second FM modulation wave are set in such a manner that the frequencies thereof are not intersected with each other.
Further, the FM-CW radar apparatus according to the present invention is characterized in that a frequency of the first FM modulation wave and a frequency of the second FM modulation wave are set in such a manner that the frequencies thereof are intersected with each other in the vicinity of each of center frequencies of the first and second FM modulation waves.